1. Technical Field
The invention relates to a color gamut forming apparatus, a color converting apparatus, a color gamut forming method, a color conversion method, a computer-readable medium storing a color gamut forming program, a computer-readable medium storing a color converting program and a computer data signal.
2. Related Art
In general, an image input device and an image output device are different in an input color gamut, which is an input or reproducible range of colors. Accordingly, when an image input by the image input device is output by the image output device, a color, being not included in an output color gamut which is a color gamut of the image output device, of colors in the input color gamut which is a color gamut of the image input device cannot be reproduced by the image output device. Therefore, it is necessary to convert a color, being not included in the output color gamut, of the colors in the input color gamut into a color in the output color gamut. Also, it is desired that color reproduction that is close to the image of the input color gamut be performed by the image output device.
There have been suggested several methods of converting colors from the input color gamut to the output color gamut. FIG. 13 is an explanatory diagram illustrating a first example of a color conversion method of a related art. In this method, the hue is not changed, but the lightness and the saturation are changed. This method is employed, for example, in JP 2002-335416 A.
In FIG. 13, a solid line indicates the contour of the input color gamut and a bold line indicates the contour of the output color gamut. FIG. 13 shows an example in which red (R) is converted. In the first example, as shown in FIG. 13B, the red (R) in the input color gamut is changed in the saturation direction and in the lightness direction, and is converted into a color indicated by R′ on the contour of the output color gamut. The color R′ is different from the red (R) in the input color gamut as shown in FIG. 13(A).
FIG. 14 is an explanatory diagram illustrating a change in color in the first example of the color conversion method of the related art. FIG. 14 shows that when colors are converted using the method shown in the first example of the related art, what colors the colors on the contour of the input color gamut are converted into. First, as shown in FIG. 14, a color is changed in a direction from the red R to magenta M along the contour of the input color gamut. As the color gets apart from the red R, the saturation and the lightness slowly decrease, and then the color is changed to magenta M while the saturation slowly increases from a certain degree. However, in the output color gamut, the saturation increases from the color R′ to the red R and then, the saturation decreases. Accordingly, the change in saturation is inverted in the vicinity of the red. Thus, the impression of the color would be different.
FIG. 15 is an explanatory diagram illustrating a second example of the color conversion method of the related art. In this method, a color is converted into a color having a small color difference from the original color. For example, this method is employed in JP 2003-153020 A to generate a common color gamut of plural color gamuts and then to perform color conversion for the common color gamut.
When the method shown in the second example is used, the change in saturation in the input color gamut is different from that in the output color gamut as described with reference to FIG. 14. As can be seen from FIG. 15B, when the saturation decreases along the contour of the input color gamut from a color having the highest saturation, the saturation slowly decreases in the input color gamut. However, in the output color gamut, the saturation once increases and then decreases. Thereby, the impression of colors is greatly changed due to the inversion of the change in color.
FIG. 16 is a diagram illustrating a third example of the color conversion method of the related art. In this method, saturated colors in the input color gamut are converted into saturated colors of the output color gamut. For example, this method is used in JP 2004-32140 A.
Saturated colors in a color gamut are colors in which one or more color components thereof are 100% and the other color components are 0%. For example, when the color components include red (R), green (G), and blue (B), the saturated colors include 100% R, 100% G and 100% B, and also include cyan (C), magenta (M), and yellow (Y) in which two of R, G and B are 100%. The saturated color may include white and black. For example, when the color components include C, M, Y, the saturated colors include C, M, Y, R, G, B and may further include white, and black. When the color components include black (K) in addition to C, M, and Y, the saturation colors may further include CK, MK, YK, RK, GK, and BK. The saturation colors usually have the local maximum saturation value in comparison with the neighboring colors, and may often form convex points in a three-dimensional device-independent color space such as an LAB color space.
In the third example, as shown in FIG. 16, the saturated colors of the input color gamut are converted into the saturated colors of the output color gamut. Accordingly, a difference in saturation change shown in FIGS. 14 and 15 does not occur. However, the saturated colors of the input color gamut may be greatly different from the saturated colors of the output color gamut. In this case, the impression of an image reproduced is changed.